1. Field of the Invention
This invention relates to signal processing of video signals reproduced from a television receiver or a VTR, and more particularly relates to a video signal contour correcting circuit for enhancing the contour of video signals.
2. Description of the Related Art
The video signal reproduced by a recording/reproducing apparatus, the video signal transmitted by a cable, or the video signal imaged by a television camera loses its high frequency component depending on the general transmission band generally, and the sharp rising waveform or the sharp falling waveform of the video signal loses the sharpness of the contour to form a rounded waveform, as the result, the sharpness of an image on a monitor screen is rounded.
An contour correcting circuit for correcting the signal contour by use of a circuit is used.
FIG. 5A is an exemplary correcting circuit for contour correction of the video signal, 21 and 22 denote differential circuit, and 23 denotes a subtraction circuit for subtracting an contour-corrected signal from an original video signal.
FIG. 5B shows the signal waveforms at respective positions on the above-mentioned circuits, (A) is the video signal waveform having the contour which forms a inflection point of the video signal rounded due to the circuit characteristics, (B) is the harmonic signal component (first order differential) of an edge formed by passing the video signal waveform (A) through the first differential circuit 21, and (C) is a quadratic differential waveform signal formed by passing further the signal of (B) through the second differential circuit 22, which (C) is resultantly the signal waveform obtained by differentiating the harmonic component twice.
The signal waveform which has been differentiated twice is provided to the subtraction circuit 23, and subtraction circuit 23 subtracts the provided signal waveform from the original video signal to form a signal having the contour with the inflection point of the video signal which rises steeply and is enhanced as shown in (D).
As described herein above, the signal which has been differentiated twice to correct the contour of the video signal has a waveform contour which rises sharply and falls sharply, however, the high level overshoot Q and preshoot P are added to the original video signal when the contour correction is enhanced, and a needless frame is resultantly added to the original video signal to form an image which seems different from the original image.
On the other hand, if the video signal is subjected to mild differentiation, the sharp contour is not obtained.